Long Term Evolution (LTE)
The 3rd Generation Partnership Program (3GPP) standardized a new mobile communication system called Long Term Evolution (LTE). LTE has been designed to meet carrier needs for high speed data and media transport as well as high capacity voice support. The work item specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) has been released as Release 8 (LTE Rel. 8).
The LTE system provides packet-based radio access and radio access networks with fully IP-based functionality at low latency and low costs. LTE specifies multiple transmission bandwidths to achieve flexible system deployment. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access is used, while single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink. Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques, and a highly efficient control signaling structure is achieved in LTE Rel. 8/9.
The frequency spectrum for IMT-Advanced (4G) was decided at the World Radio communication Conference 2007 (WRC-07). IMT-Advanced, which includes LTE-Advanced (also known as LTE-A or LTE Rel. 10), provides a global platform on which to build next generations of interactive mobile services that will provide faster data access, enhanced roaming capabilities, unified messaging and broadband multimedia. The specification of LTE-A introduced enhancements such as carrier aggregation, multi-antenna enhancements and relays (Relay Nodes). The 3GPP specification of LTE-A was finalized in March 2011 and supports peak data rates up to 3.5 GBit/s in the downlink and 1.5 Gbit/s in the uplink. Further, LTE-A introduces support of Self Organizing Networks (SON), Multimedia Broadcast/Multicast Service (MBMS) and Heterogenous Networks (HetNets). Other LTE-A enhancements to LTE include architecture improvements for Home (e)NodeBs (i.e. femtocells), local IP traffic offloading, optimizations for machine-to-machine communications (M2C or MTC), SRVCC enhancements, eMBMS enhancements, etc.
In December 2012, further improvements to LTE-A have been standardized in the 3GPP in LTE-A Rel. 11. With this at present latest release of LTE-A, features like Coordinated Multi-Point transmission/reception (CoMP), Inter-Cell Interference Coordination (ICIC) enhancements, Network Improvements for Machine-Type Communication (NIMTC), etc.
LTE Architecture
FIG. 1 exemplarily shows the architecture of LTE, which equally applies to LTE-A as well. FIG. 2 illustrates the E-UTRAN architecture in more detail. The E-UTRAN comprises the eNodeB (which can be also referred to as a base station). The eNode B provides the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. The eNodeB is also responsible for handling Radio Resource Control (RRC) functionality corresponding to the control plane and also implements several additional management functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (S-GW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The S-GW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies. The S-GW terminates the S4 interface and relays the traffic between 2G/3G systems (via SGSN) and the PDN GW (P-GW). For idle state UEs, the S-GW terminates the downlink data path and triggers paging when downlink data arrives for the UE. It manages and stores UE contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode UE tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the S-GW for a UE at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs. It checks the authorization of the UE to camp on the service provider's Public Land Mobile Network (PLMN) and enforces UE roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming UEs.
LTE-A Improvements for Machine Type Communications (MTC)
Machine Type Communications (MTC) refers to communications between machines (typically MTC applications running on a hardware and communicating with each other) through mobile communication networks or other types of networks. In the 3GPP context, a MTC device denotes a UE equipped for machine type communications (sometimes also referred to as MTC UE), which communicates with MTC Server(s) and/or other MTC device(s). A MTC server can be considered an entity, which communicates to MTC devices e.g. through a PLMN.
An example of MTC technology might be a set of devices that monitor traffic in a city and communicate the information to the city's traffic lights in order to regulate the flow of traffic. MTC could be for example used in telemetry, data collection, remote control, robotics, remote monitoring, status tracking, road traffic control, offsite diagnostics and maintenance, security systems, logistic services, fleet management, and telemedicine.
MTC is expanding rapidly and has the potential to generate significant revenues for mobile network operators. MTC Devices are expected to outnumber voice subscribers by at least two orders of magnitude. Some predictions are much higher. MTC can enable machines to communicate directly with one another. MTC communication has the potential to radically change the world around us and the way that people interact with machines.
Machine type communications are expected to be one of the most differentiating technologies and applications in (building) next-generation communications networks (i.e. 5G). In addition to ultra-high network speed and increased maximum throughput (as compared to 4G), 5G technologies are expected to provide efficient support of machine-type devices to enable the Internet of Things with potentially higher numbers of connected devices, as well as novel applications such as mission critical control or traffic safety, requiring reduced latency and enhanced reliability.
3GPP TR 37.868, “RAN Improvements for Machine-type Communications”, Version 11.0.0 (available at http://www.3gpp.org) studied the traffic characteristics of different MTC applications with machine-type communications and define new traffic models based on these findings. In this context, radio enhancements for UTRAN and E-UTRAN to improve the support of machine-type communications were also studied.
FIG. 3 illustrates the roaming architecture for 3GPP Architecture for Machine-Type Communication for a so-called home routed scenario, as it is specified in 3GPP TR 23.888, “System improvements for Machine-Type Communications (MTC)”, version 11.0.0, (available at http://www.3gpp.org). In a so called Direct Model, the MTC Application communicates with the UE for MTC directly as an over-the-top application on 3GPP network. In this model, as illustrated in FIG. 4, the signaling (control plane) of MTC applications running on UEs or MTC devices located in the E-UTRAN is interfaced by a MTC InterWorking Function (MTC-IWF) which typically resided in the Home-PLMN. As further shown in FIG. 5, the user plane traffic of carrying the MTC data is relayed through the P-GW (optionally via the MTC Server) to the target MTC application (or vice versa). The MTC application may be running on the MTC server or another device inside or outside the home PLMN. An MTC-IWF could be a standalone entity or a functional entity of another network element. The MTC-IWF hides the internal PLMN topology and relays or translates signaling protocols used over MTCsp towards the MTC Server to invoke specific functionality in the PLMN. In the Direct Model, the MTC data is transmitted through the 3GPP network.
The RAN improvements should enable or improve the usage of RAN resources efficiently, and/or reduce the complexity when a large number of machine-type communications devices possibly need to be served based on the existing features as much as possible. Meanwhile, minimize the changes of existing specifications and the impaction of Human-to-Human (H2H) terminals in order to keep the complexity related to M2M optimizations at a minimum level. An overview of MTC enhancement and other enhancement for mobile data applications at the 3GPP for LTE-A (Release 12) is provided in 3GPP 23.887, “Machine-Type and other mobile data applications Communications enhancements”, version 1.3.0 (available at http://www.3gpp.org).
PDN Connectivity and MTC
Connectivity for MTC data transmission is based on packet data network (PDN) connectivity. In LTE-A, a PDN connection is an association between an UE and a PDN GW (P-GW). It is represented by one IPv4 address and/or one IPv6 prefix of the UE. As shown in FIG. 3, P-GW is the gateway for MTC data to and from the MTC Server or the UE. Generally, a UE may have simultaneous connectivity with more than one P-GW for accessing multiple packet data networks.
For E-UTRAN access to the EPC, the PDN connectivity service is provided by an EPS bearer between the UE and the P-GW for a GTP-based S5/S8 interface, and by an EPS bearer between UE and S-GW concatenated with IP connectivity between S-GW and P-GW for PMIP-based S5/S8. An EPS bearer uniquely identifies traffic flows that receive a common Quality of Service (QoS) treatment between a UE and a P-GW for GTP-based S5/S8 and between UE and S-GW for PMIP-based S5/S8. The packet filters signaled in the NAS procedures are associated with a unique packet filter identifier on per-PDN connection basis.
One EPS bearer is established when the UE connects to a PDN, and that remains established throughout the lifetime of the PDN connection to provide the UE with always-on IP connectivity to that PDN. That bearer is referred to as the default bearer. Any additional EPS bearer that is established for the same PDN connection is referred to as a dedicated bearer.
An UpLink Traffic Flow Template (UL TFT) is the set of uplink packet filters in a TFT. A DownLink Traffic Flow Template (DL TFT) is the set of downlink packet filters in a TFT. Every dedicated EPS bearer is associated with a TFT. A TFT may be also assigned to the default EPS bearer. The UE uses the UL TFT for mapping traffic to an EPS bearer in the uplink direction. The PCEF (for GTP-based S5/S8) or the BBERF (for PMIP-based S5/S8) uses the DL TFT for mapping traffic to an EPS bearer in the downlink direction.
As illustrated in FIG. 6, traffic flows are mapped onto the corresponding PDN connections using TFTs (Traffic Flow Templates).
PDN connectivity is established through the following procedures:    1. Establish the RRC connection between UE and eNodeB, using RRC connection establishment procedure;    2. If UE is in EMM-IDLE mode, the Service Request procedure is performed before the PDN Connectivity procedure can be initiated;    3. The PDN Connectivity Request procedure is initiated by UE towards MME;    4. PDN connection requests are rejected immediately if any request of the above procedures cannot be accepted by the network.
With the current concept of PDN connectivity, one PDN connection is exclusively used by a particular UE. Accordingly, as shown in FIG. 7, multiple MTC UEs will generate a plurality of different types of PDN connections.
MTC traffic typically exhibits significantly different characteristics from conventional mobile traffic initiated by subscribers, including:    1. Non-interactive: machine initiated traffic; non-real-time; delay tolerant (depending upon application requirements); potentially time-shift-able.    2. Short connectivity duration: sporadic connectivity (i.e. active for traffic for much less time than smart phones; occurring much less frequently).    3. High signaling overhead: disproportional amount of control plane overhead, compared to user plane traffic, for establishing and tearing down short sessions.    4. Dominant uplink traffic volume: much larger ratio of uplink to downlink traffic volume; requiring low latency in uplink; smaller packets in size.    5. Bursty traffic aggregate: synchronized traffic resulting in bursty aggregate signaling traffic/session volumes.    6. Predictable mobility: mobility depending upon types of MTC devices; more predictable mobility for the same type of MTC devices.
Existing radio access networks are mainly designed for continuous data traffics in the downlink, and optimized for high downlink data rate/volume, which in turn makes control signaling overhead manageable. Further, they are mainly designed for instantaneous communications for human initiated at-will connectivity requests, which have to be rejected immediately if network resources (at the time of receiving the requests) cannot satisfy the requests.
In comparison, MTC applications may be expected to be non-interactive, demanding more uplink capacity and introducing disproportional amount of control plane overhead (in establishing and tearing down short sessions). A large number of MTC devices is expected to be deployed in a specific area, thus the network may have to face increased load as well as possible surges of MTC traffic, especially the signaling traffic.
Network congestion including radio network congestion and signaling network congestion (at core nodes) may happen due to mass concurrent data and signaling transmission. This may significantly downgrade network performance and affect quality of experience of smart phone users, leading to dead connections, dropped calls, bad coverage, and intermittent data connectivity.
Mechanisms to guarantee network availability and help networks to meet performance requirements under such MTC load need to be further investigated. One promising way is to optimize protocols and system design of mobile access networks, based on characteristics of MTC traffic, in order to accommodate large volume of MTC devices in existing networks.